The useful physical properties of vulcanizates of elastomers are strongly dependant on the nature, concentration and the extent of interaction of the carbon black fillers and the elastomer. These physical properties are exemplified by tensile strength, abrasion resistance, modulus of elongation and resistance to solvents. At decreasing concentrations of carbon black in the elastomer phase these properties degrade rapidly and are essentially useless for the manufacture of articles of commerce.
Physical and mechanical properties of elastomeric materials which cannot be achieved with any one available polymer can be approached by using blends of different polymers. Such blends are known and common examples are blends of butyl rubber with styrene-butadiene rubber or ethylene-propylene diene (EPDM) rubber with acrylate rubbers. Typically these blends will use a combination of rubbers with very different characteristics to approach the desired performance properties. Thus for example a saturated or a slightly unsaturated nonpolar hydrocarbon rubber will be blended with a more unsaturated and/or more polar elastomer (hereinafter "polar/nonpolar elastomer blends"). Typical examples of the saturated or slightly unsaturated elastomer are the following: ethylene-propylene copolymer, polyisobutylene, ethylene-propylene-diene rubber and isobutylene-isoprene rubber. Typical examples of the unsaturated and/or polar elastomers are nitrile-butadiene rubbers, acrylic rubbers, polybutadiene rubbers, styrene-butadiene rubbers, natural rubber and chlorinated polyisoprene.
In blends of two or more of the above elastomers chosen from each of the above categories the development of acceptable physical properties requires the mixing of the blend with carbon black prior to vulcanization. These elastomers are not soluble in each other and exhibit discrete multiphase morphologies when blended. The phase size of each elastomer ranges from submicrometer to several micrometer depending on the composition of the rubber, mixing conditions and their relative viscosities. When carbon black is introduced into these mixtures it has been observed that the carbon black will be nonuniformly distributed among the different elastomers and will concentrate in the more polar/unsaturated elastomer. For example see, Callan, Topcik and Ford in Rubber World 151, 60 (1965) and Marsh, Mullens and Price in Rubber Chem and Tech 43, 400 (1970).
The properties of the vulcanizates of these polar/nonpolar elastomer blends are usually significantly worse than expected values of these properties based on a simple compositionally weighted arithmetic average of the properties of the vulcanized constituent elastomers. As an example, Schuster, Schmidt and Pampus in the journal Kautshuk. Gummi. Kunstst vol 42, p 590 (1989) have described extensive investigations into the formation of vulcanized and unvulcanized blends of nitrile-butadiene rubber (NBR) and EPDM rubber and have concluded that these NBR/EPDM blends do not have the physical, mechanical and use properties expected from a simple weighted arithmetic average of the properties of the two elastomers. As a further example Sircar in Rubber Chem. and Tech. 47, 48 (1974) showed that blends of chlorobutyl rubber and polybuatdiene were characterized by a depletion of carbon black from the chlororbutyl phase and poor physical and mechanical properties of the final vulcanizates.
This unequal distribution of carbon black in polar non-polar elastomer blends and in a single elastomer itself causes uneven physical properties throughout the elastomeric blend. More importantly the bulk properties of such a blend of elastomers reflect those of the weakest component which is the saturated or nearly saturated, nonpolar hydrocarbon polymers deficient in the amount of carbon black. Thus it is highly desirable that carbon black distribution be dispersed in both phases in blends of polar/non-polar elastomers and even more advantageously, controllable.
Previous efforts to control carbon black distribution in polar/non-polar elastomer blends have relied on intricate compounding procedures involving the use of premixed separate masterbatches of nitrile-butadiene rubber and polybutadiene rubber which are then blended together, or using an oil to disperse the carbon black in pre-blends of nitrile-butadiene rubber and polyisobutylene rubber or in blends of nitrile-butadiene rubber and styrene-butadiene rubber. These procedures do not produce elastomer blends containing a uniform or controllable distribution of carbon black in the different elastomer phases. Sircar and Lamond in Rubber Chem. and Tech. 46, 178 (1973) showed carbon black which had been introduced exclusively into chlorobutyl rubber by a preliminary mixing step would migrate away from the saturated elastomer into the more unsaturated elastomer. They also found that this migration could only be retarded or eliminated by increasing the interaction of carbon black and the chlorobutyl rubber either by a thermal or chemical procedures prior to the introduction of the other elastomer. Thus, a need exists in the art to provide a method for controlling carbon black distribution in elastomers and elastomer blends.